Apparatus for producing oriented plastic strap

ABSTRACT

An apparatus for producing an oriented plastic strap having a predetermined desired thickness for use in connection with the strapping of packages or the like. Heating is provided in connection with the surface of the sheet workpiece which is exposed to the ambient air prior to entry of the workpiece into the nip defined between the pair of rollers such that both surfaces will have similar temperature levels so as to exhibit similar density properties, and still further, the edge portions of the sheet workpiece are also heated so as to advantageously control the degree of flatness of the sheet material or workpiece across the width thereof. The flatness quality of the sheet workpiece can also be achieved through means of specially contoured milling rollers defining the zero gap assembly nip, or by means of specially contoured extrusion dies, whereby the edge portions are rendered thinner as compared to the central portion of the sheet workpiece such that upon milling and stretching of the workpiece, which operations tend to thicken the edge portions of the sheet workpiece, the edge portions will exhibit thickness dimensions commensurate with the thickness dimension of the central portion of the sheet workpiece.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This application is a continuation of application Ser. No. 08/528,282,filed Sep. 14, 1995, now abandoned, which is a division of Ser. No.08/353,721 filed Dec. 12, 1994, now U.S. Pat. No. 5,525,287 which is aCIP of Ser. No. 07/958,803 filed on Oct. 9, 1992, now U.S. Pat. No.5,387,388.

FIELD OF THE INVENTION

The present invention relates generally to a method and apparatus forproducing an oriented plastic strap, and a strap produced ed by suchmethod and apparatus, and more particularly to a method and apparatusfor simultaneously milling and stretching a plastic sheet into strapstock material having a predetermined desired thickness.

BACKGROUND OF THE INVENTION

In accordance with a typical prior art stretching process, such as, forexample, the SIGNODE process, a cast sheet of thermoplastic material,such as, for example, polypropylene, is first reduced in size, that is,the thickness dimension thereof, by rolling the sheet through a pair ofclosely spaced milling rollers or cylinders which rotate in oppositedirections. After the thickness of the sheet has been reduced, the sheetis then drawn and stretched out from the milling rollers by means of aseries of orienting rollers or a bridle assembly so as to achieve itsfinalized desired size or thickness.

Another prior art process or method that is commonly used in connectionwith the fabrication of stretched sheet materials is a process known asor called the short-gap method and is generally comprised of an entrybridle, a stretching assembly, and an exit bridle. In accordance withthis process, a slow speed, heated entry bridle assembly advances a castsheet of material, usually film, to a stretching assembly whichcomprises a pair of rollers or cylinders which are set a predetermineddistance apart. The first roller rotates at the same speed as the entrybridle, whereas the second roller rotates at a speed which is greaterthan that of the first roller and which is equal to the rotary speed ofthe exit bridle. Thus, as the film passes through the entire assembly,it is stretched to its finalized desired size or thickness dimension.

These prior art methods or processes present several disadvantages. Forexample, the properties of the straps produced by these methods orprocesses provide or exhibit limited increases in strength withoutsignificant increases in other desired properties. In addition,substantial necking of the sheets occur as the sheets are stretched overthe distance or space defined between the rollers.

The aforenoted parent patent application presented, disclosed, andembodied a novel process and apparatus for achieving the simultaneousmilling and stretching of such sheets whereby the aforenoted problemswere minimized, however, other operational problems with such processand apparatus have now been discovered which require rectification inorder to achieve still better properties characteristic of suchsimultaneously milled and stretched sheets of thermoplastic material.For example, one operational problem or deficiency which has beendiscovered in connection with the fabrication or manufacture of thesimultaneously milled and stretched sheets of thermoplastic material inaccordance with the zero-gap process and apparatus set forth anddescribed within the aforenoted parent patent application is that whenthe thermoplastic sheet is ready to be worked, that is, simultaneouslymilled and stretched, by being conducted through the nip defined betweenthe oppositely rotating rollers of the zero-gap assembly, it has beenfound that the opposite surfaces of the thermoplastic sheet exhibitdifferent surface temperature values. This is because as thethermoplastic sheet is routed about the first or upstream roller of thezero-gap roller assembly, the inside surface of the thermoplastic sheet,that is, the surface of the sheet which is in direct contact with thefirst or upstream roller of the roller assembly, will be heated by thefirst or upstream roller, or at least have its surface effectivelyprevented from being cooled by the ambient air because such surface isdisposed in direct contact with the first or upstream roller and notexposed directly to the ambient air. On the contrary, the outer orexterior surface of the thermoplastic sheet, that is, the surface of thesheet which is not disposed in direct contact with the first or upstreamroller, is effectively cooled as a result of being exposed to theambient air. Such temperature differential between the aforenotedsurfaces of the thermoplastic sheet leads to density differentialsthroughout the thermoplastic sheet which adversely affects the variousproperties of the processed sheets, such as, for example, the tensilestrength, weldability and associated properties, and split resistance.

Another problem or deficiency which has been discovered in connectionwith the aforenoted zero-gap simultaneous milling and stretchingapparatus and method is that as a result of the processing of thethermoplastic sheet, the opposite edge portions of the sheet becomethickened, or in other words, the sheet does not exhibit it uniformthickness or flatness across the width thereof. Consequently, when theprocessed sheet is subsequently desired to be processed further intothermoplastic strapping, the thickened edge portions cannot be used tofabricate such straps, at least without further processing in order toeffectively reduce the thickness dimension to the desired thicknessdimension characteristic of the useable strapping. Consequently, it isdesired to laterally extend or increase the width of the zone or regionof the processed sheet which exhibits the uniform thickness dimension orflatness of the sheet so as to effectively increase the amount ofprocessed sheet which can then be readily fabricated into the desiredstrapping.

OBJECTS OF THE INVENTION

Accordingly, it is an object of the present invention to provide a newand improved method and apparatus for producing oriented plastic strap,and an oriented plastic strap produced by such method and apparatus.

Another object of the present invention is to provide a new and improvedmethod and apparatus for producing oriented plastic strap, and anoriented plastic strap produced by such method and apparatus, whichovercomes the various disadvantages, problems, ant deficiencies of theprior art processes and apparatus.

Yet another object of the present invention is to provide new andimproved method and apparatus for producing oriented plastic strap whichis capable of simultaneously milling and stretching a sheet workpieceinto a strap which has a predetermined thickness dimension as a resultof the sheet workpiece being conducted in a single pass through a nipdefined between a pair of zero-gap rollers.

Still another object of the present invention is to provide a new andimproved method and apparatus for producing oriented plastic strap, andan oriented plastic strap produced by such method and apparatus, whichhas significantly increased tensile strength and split resistance whencompared to straps produced by known prior art methods and apparatus.

A further object of the present invention is to provide a new andimproved method and apparatus for producing oriented plastic strap, andan oriented plastic strap produced by such method and apparatus, whichexhibits high tensile strength, high split resistance, and improvedwelding characteristics.

A yet further object of the present invention is to provide a new andimproved method and apparatus for producing oriented plastic strap, andan oriented plastic strap produced by such method and apparatus, whereinuniform density characteristics are imparted to the sheet workpiece suchthat the milled and stretched strap produced within the zero-gap rollerassembly exhibits improved tensile strength, weld, and split resistanceproperties.

A still further object of the present invention is to provide a new andimproved method and apparatus for producing oriented plastic strap, andan oriented plastic strap produced by such method and apparatus, whereinimproved uniform thickness and flatness characteristics are imparted tothe finalized milled and stretched strap as a result of various heating,rolling, and extrusion techniques being imparted to the original sheetworkpiece-or material.

SUMMARY OF THE INVENTION

Briefly, the foregoing and other objects are achieved in accordance withthe present invention through the provision of a method and apparatusfor producing an oriented plastic strap which will have a predetermineddesired thickness for use in strapping packages and the like wherein thestrap, having the noted predetermined desired thickness, issimultaneously milled and stretched as a result of a plastic sheetworkpiece being passed through a nip defined between a pair of rollersor cylinders which are spaced closely together with respect to eachother. The nip has a space dimension which is substantially less thanthe original thickness dimension of the sheet workpiece, and the rollersare rotated in opposite directions and at substantially different linealsurface velocities.

In order to improve the various properties of the simultaneously milledand stretched strap, the density of the sheet workpiece, prior to entryinto the aforenoted nip defined between the pair of milling rollers, isrendered more uniform across the thickness of the sheet workpiece byheating the outer surface of the sheet workpiece to an elevatedtemperature, that is, to a temperature level which is greater than thatof the inner surface of the sheet workpiece which is disposed in contactwith one of the milling rollers, such that despite the cooling of theouter surface of the sheet workpiece as a result of the exposure of theouter surface of the sheet workpiece to the ambient air, the inner andouter surfaces will exhibit substantially the same temperature levelstherefore substantially uniform densities.

In order to improve the flatness of the resultant simultaneously milledand stretched sheet such that increased volumetric strapping can beproduced from such sheeting, edge heaters are disposed along theopposite edge regions of the sheet workpiece prior to entry of the sheetworkpiece into the nip defined between the zero-gap milling rollers.Such edge heating has been found to significantly reduce the width ofthickened edge portions of the milled and stretched sheet, as measuredinwardly from the edge portions the sheet, such that an increased amountof useable sheet can be produced while concomitantly resulting inreduced amount of sheeting which must be trimmed and discarded.

Another manner in which increased flatness of the milled and stretchedsheeting can be achieved is to initially impart to the sheet workpiece,either upstream or prior to entry of the sheet workpiece into the nipdefined between the mill rollers, or within the nip defined between themiller rollers, thinner edge portions. Consequently, in view of thetendency of the edge portions of the workpiece to become thickened as aresult of the simultaneous milling and stretching of the sheetworkpiece, the edge portions will exhibit thickness dimensions whichwill be commensurate with the thickness dimension of the central portionof the sheet workpiece, as viewed transversely across the width of thesheet workpiece, when the sheet workpiece is simultaneously milled andstretched. Production of the thinner edge portions of the sheetworkpiece can be produced, for example, by means of specially contouredmilling rollers employed within the zero-gap system or assembly definingthe nip through which the sheet workpiece is forced to pass, thezero-gap mill rollers effectively defining a nip therebetween which hasa trapezoidal cross-sectional configuration. In a similar manner, and inaccordance with a second means or mode for producing thinner edgeportions upon the sheet workpiece, specially contoured extrusion diesand casting rollers can be employed so as to similarly provide theextruded and cast sheet workpiece with a substantial trapezoidalconfiguration such that the edge portions of the sheet workpiece aretapered. As a result of the simultaneously milling and stretching of thesheet workpiece, the tapered edge portions thereof will experienceincreased thickening whereby, as a result, the thickness dimensions ofthe edge portions will become similar to the thickness dimension of thecentral portion of the sheet workpiece.

BRIEF DESCRIPTION OF THE DRAWINGS

Various other objects, features, and attendant advantages of the presentinvention will become more fully appreciated from the following detaileddescription, when considered in connection with the accompanyingdrawings, in which like reference characters designate like orcorresponding parts throughout the several views and wherein:

FIG. 1 is a fragmentary front view of apparatus, constructed inaccordance with the present invention, for producing oriented plasticstrap in accordance with the zero-gap manufacturing process of thepresent invention;

FIG. 2 is an enlarged, simplified, fragmentary front view of the millingand stretching rollers defining the zero-gap assembly of FIG. 1;

FIG. 3 is an enlarged, partial, cross-sectional view of the milling andstretching rollers defining the zero-gap assembly of FIG. 1 as takenalong the line 3--3 of FIG. 1;

FIG. 4 is a simplified, fragmentary cross-sectional view on a reducedscale, of the milling and stretching rollers defining the zero-gapassembly of FIG. 1, as taken along a line or direction parallel to line3--3 of FIG. 1;

FIG. 5 is a front view of apparatus, constructed in accordance with thepresent invention and similar to the apparatus shown in FIG. 1, showing,however, a modified embodiment for producing oriented plastic strap inaccordance with the zero-gap manufacturing process of the presentinvention wherein the sheet workpiece has its external surface heatedprior to entry into the zero-gap roller assembly, as well as its edgeportions heated prior to entry into the nip defined between the millingand stretching rollers of the zero-gap roller assembly;

FIG. 6 is a schematic drawing of the processing line used in conjunctionwith the zero-gap milling and stretching roller assembly whereintrimming apparatus is utilized to trim the thickened edge portions ofthe milled and stretched strap prior to rolling of the finished straponto storage rolls;

FIGS. 7A and 7B are front views of two different embodiments of millingrollers which can be used within the zero-gap assembly of either FIGS. 1or 5 so as to produce milled and stretched plastic strap which hasthinner edge portions such that the amount of useable sheetingexhibiting the desired degree of flatness, as viewed in the transversedirection across the width of the sheeting, is enhanced;

FIGS. 8A and 8B are diagrammatic cross-sectional views of extrusion dieswhich can be used to produce a sheet workpiece having thinner edgeportions such that the amount of useable sheeting exhibiting the desireddegree of flatness, as viewed in the transverse direction across thewidth of the sheeting, when the sheeting is milled and stretched, isenhanced; and

FIG. 9 is a front view of casting rolls used in conjunction with theextrusion dies of FIGS. 8A and 8B for casting and further forming thesheet workpiece extruded from the extrusion dies of FIGS. 8A and 8B suchthat the sheet workpiece can be conducted downstream toward the zero-gapmilling and stretching assembly.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

Referring now to the drawings, and more particularly to FIG. 1 thereof,there is schematically illustrated a zero-gap roller assembly, generallyindicated by the reference character 20 for simultaneously milling andstretching or elongating a sheet workpiece 22 into a thin strap stockmaterial. The present invention is discussed and illustrated with only asingle sheet workpiece 22 being conducted through the assembly 20,however, it is to be understood that more than one sheet or workpiece 22may be simultaneously passed through the assembly 20. It is to befurther noted that the phrase or terminology "zero-gap" as usedthroughout this patent application refers to the concept ofsubstantially eliminating any gap between the step of milling the sheetworkpiece and the step of stretching the sheet workpiece. In otherwords, the steps of milling and stretching of the sheet workpiece areaccomplished substantially simultaneously. In addition, it is noted, asdisclosed in FIG. 1, that the zero-gap assembly 20 is located between asheet or workpiece feeding assembly 24 and an exit bridle assembly 26which are located or mounted upon a frame or support 28.

The feeding assembly 24 may take any one of several forms, and as shownin FIG. 1, comprises an extruding machine 30 for extruding a sheet orworkpiece 22 of stock material, and an entry bridle assembly 32. Theextruding machine 30 produces a sheet piece 22 from a suitable material,such as, for example, polypropylene or the like, and the sheet workpiece22 is conducted into the entry bridle assembly 32 which, in turn, feedsthe sheet workpiece into the zero-gap roller assembly 20. The sheetworkpiece 22 may be heated during its passage through the entry bridleassembly 32 so as to in effect be pre-heated upon entry into thezero-gap assembly 20 in order to enhance the working properties of thematerial comprising the sheet workpiece 22.

The entry bridle assembly 32 is seen to comprise a plurality of rollersor cylinders 34, 36, 38, and 40 which are mounted by suitable means,such as, for example, shafts, not shown, upon the frame or support 28.The rollers 34, 36, 38, and 40 may be either solid or hollow, and in thepreferred embodiment, as illustrated in FIG. 1, the rollers 34, 36, 38,and 40 are essentially only used to properly deliver or feed the sheetworkpiece 22 into the zero-gap assembly 20, and they do notsubstantially contribute to any stretching or milling of the sheetworkpiece 22. A different number of rollers may be employed than thatshown in FIG. 1, and it os seen that the rollers 34, 36, 38, and 40 arearranged within two vertically spaced rows with the bottom row rollers36 and 40 being located between or offset with respect to the top rowrollers 34 and 38. Rollers 34 and 38 are also mounted for rotation inthe clockwise direction while rollers 36 and 40 are mounted for rotationin the counterclockwise direction, whereby when the sheet workpiece 22is wound around or routed through the entry bridle assembly 32, ittravels through the rollers 34, 36, 38, and 40 in the proper mode ordirection with respect to the directions of rotation of the rollers 34,36, 38, and 40. Each one of the rollers 34, 36, 38, and 40 are rotatedat a uniform speed by suitable means, not shown, such as, for example, amotor and shaft drive assembly, and all of the rollers 34, 36, 38, and40 rotate at substantially the same speed or lineal surface velocity asthe top roller 42 of the zero-gap assembly 20, which will be discussedin greater detail hereinafter.

Continuing further, after the sheet workpiece 22 passes through thefeeding assembly 24, it advances toward the zero-gap assembly 20 forsimultaneous milling and stretching into a finished sheet 22 which has apredetermined desired thickness. The zero-zap assembly 20 comprises apair of rollers or cylinders 42 and 44 that are rotatably mounted in anopposing relationship. The nip 46, that is, the distance defined betweenthe rollers 42 and 44, can be substantially varied depending upon thedesired finished thickness of the finished sheet 22. The zero-gaprollers 42 and 44 may either be solid or hollow, and may be heated byany well-known means, not shown, such as, for example, circulating aheated fluid through the rollers 42 and 44, in order to enhance thestretching properties of the sheet material. The zero gap rollers 42 and44 may also be flat, as shown in FIGS. 1-4, or may be contoured, asshown in FIGS. 7A and 7B, in order to change the shape of the sheetworkpiece 22 as it passes through the rollers 42 and 44 of the zero-gapassembly 20, as will be more fully discussed hereinafter.

As best shown in FIG. 2, the upper roller 42 is driven a clockwisedirection, as shown by means of its arrow, and the bottom or lowerroller 44 is driven in the counterclockwise direction as similarly shownby means of its arrow. Thus, the sheet workpiece 22 is first routedaround a major circumferential portion of the top or upper roller 42,the workpiece 22 is then conducted through the nip 46 defined betweenthe rollers 42 and 44, and is then conducted around a majorcircumferential portion of the bottom or lower roller 44. Moreparticularly, it is appreciated that the sheet workpiece 22 is disposedin surface Contact with each one of the rollers 42 and 44 over acircumferential extent which is greater than one-half of each of thecircumferential dimensions of each one of the rollers 42 and 44, and itis further noted that as a result of this particular routing of thesheet workpiece 22 through the nip 46 defined between the rollers 42 and44, and about the outer circumferential or peripheral surfaces of therollers 42 and 44, each roller 42 and 44 is disposed in contact with anopposite surface or side of the sheet workpiece 22.

In accordance with the preferred, illustrated embodiment of the presentinvention, as shown in FIGS. 1-4 and as has been described in detail,the milling and stretching rollers 42 and 44 of the zero-gap assembly 20are disposed in a top-bottom or upper-lower arrangement with respect toeach other, however, it is to be appreciated that the rollers 42 and 44may alternatively be disposed in a side-by-side arrangement in whichcase the top or upper roller 42 will be disposed toward the left of thebottom or lower roller 44 such that the roller 42 will be the firstroller that the sheet workpiece 22 contacts while the bottom or lowerroller 44, which is now, in effect, the right roller, will be the secondroller that the sheet workpiece 22 contacts. As best seen in FIG. 2, therollers 42 and 44 are respectively mounted upon rotary shafts or axles52 and 54.

With particular reference being made to FIG. 4, the drive system for therollers 42 and 44 is disclosed therein, and it is seen that the axles orshafts 52 and 54 of the rollers 42 and 44 are operatively connected,through means of bearings 69 and 68, to driven shafts 48 and 50.Suitable drive means 56 and 58, such as, for example, electric motors,are mounted upon the support 28 and drive the rollers 42 and 44,respectively, through means of drive shafts 60 and 62 which areconnected to the shafts 48 and 50 by means of suitable couplings 64 and66, coupling 66 preferably comprising a universal coupling for reasonsto become apparent shortly. The bottom or lower roller 44 is connectedto the support 28 through means of bearings 68 and 70, and the coupling66, and bearings 68 and 70, permit the bottom or lower roller 44 to movewith respect to the support 28 by means of actuators 72 and 74. In thismanner, the bottom or lower roller 44 is able to be moved toward or awayfrom the stationary top or upper roller 42 so as to desirably change thesize of the nip 46 defined between the rollers 42 and 44. Each one ofthe driven shafts 48 and 50 is independently by its separate drive means56 and 58, and the bottom or lower roller 44 is driven at a speedgreater than that of the top or upper roller 42. In particular, inaccordance with the teachings of the present invention, the lower orbottom roller 44 is driven in such a manner that its lineal surfacevelocity is preferably within the range of seven to twelve (7-12) timesgreater than the lineal surface velocity of the top or upper roller 42.

In view of the foregoing, it can therefore be appreciated that as thesheet workpiece 22 is passed through the nip 46 defined between theupper and lower rollers 42 and 44 of the zero-gap assembly 20, the topor upper roller 42 effectively operates as a brake, and the millreduction operation per se may also effectively operate as a brake, uponthe lower surface of the sheet workpiece 22, that is, the surfacedisposed in contact with the upper roller 42, while the bottom or lowerroller 44 pulls and accelerates the sheet workpiece 22, the roller 44effectively operating upon the upper surface of the sheet workpiece 22,that is, the surface thereof which is disposed in contact with theroller 44. As the sheet workpiece accelerates through the nip 46 definedbetween the rollers 42 and 44, it is simultaneously milled and stretchedto its final predetermined thickness which may in fact be less than thespace dimension of the nip 46 as defined between the rollers 42 and 44.In particular, the thickness of the finished sheet 22 depends upon thelineal surface velocity differential defined between the top roller 42and the bottom roller 44, that is the faster the bottom or lower roller44 rotates relative to the top or upper roller 42, the thinner thefinished sheet 22 will be. It is also to be noted that some stretchingmay occur slightly before or after the nip 46 defined between therollers 42 and 44 depending, again, upon the surface velocity of thebottom roller 44. Thus, as has been noted hereinabove, there isessentially a zero gap between the milling and stretching functions oroperations performed in connection with the sheet workpiece 22 and theprocessing thereof into the finalized or finished sheet 22. As a resultof such simultaneous milling and stretching processing, it has beennoted that there is substantially less necking of the sheet width ascompared to the parameters of sheets which have undergone stretchingmethods in accordance with prior art processes or techniques wherein thesheet is stretched only after the milling step has been completed.

After the finished sheet 22 exits from the zero gap assembly 20, it isconducted through the exit bridle assembly 26. The exit bridle assembly26 may take any one of several different forms, and as shown in FIG. 1,the assembly 26 preferably comprises a plurality of rollers or cylinders76, 78, 80, 82, 84, and 86 which are mounted by suitable means, such as,for example, shafts or axles, not shown, upon the support 28. Theassembly 26, and in particular, the rollers 76-86, are used to pull thefinished sheet 22 out from the zero gap assembly 20. The rollers 76, 78,80, 82, 84, and 86 may be solid or hollow, and more or fewer rollersthan those disclosed may be used The rollers 76, 78, 80, 82, 84, and 86do not substantially contribute to any stretching of the finished sheet22, and as seen in FIG. 1, are arranged in two vertically spaced rowswith the bottom or lower row rollers 78, 82, and 86 also being spacedbetween the top or upper row rollers 76, 80, and 84 as considered in thetransverse direction. The upper rollers 76, 80, and 84 all rotate in theclockwise direction, while the lower rollers 78, 82, and 86 all rotatein the counterclockwise direction such that the finished sheet 22 can beproperly conducted through the exit bridle assembly 32. The rollers 76,78, 80, 82, 84, and 86 are rotated at a uniform rate of speed bysuitable drive means, not shown, such as, for example, a motor and shaftassembly, and the speed of the rollers 76, 78, 80, 82, 84, and 86 issuch that the lineal surface velocities thereof are essentially the sameas that of the bottom or lower roller 44 of the zero gap assembly 20.

It is to be appreciated, and as will be briefly noted and illustratedhereinafter, that in accordance with further teachings of the presentinvention, another stretching process and apparatus for implementing thesame, such as, for example, a short gap stretching apparatus, system, orassembly, may be used either before or after the zero gap assembly 20 soas to further modify or enhance the characteristics of the finishedsheet 22.

Having now disclosed the specifics of the apparatus comprising thepresent invention, the method to be practiced by means of such apparatusand in accordance with the present invention will now be described. Moreparticularly, the sheet workpiece 22 is fed from the extruding machine30 to the entry bridle assembly 32 and is wound around the entry bridleassembly rollers 34, 36, 38, and 40 so as to be, in turn, properlyconducted into the zero gap assembly 20. The sheet workpiece 20 is thenfed around the top or upper roller 42 of the zero gap assembly 20, thetop or upper roller 42 of the assembly 20 being driven at the samelineal surface velocity as that of the entry bridle rollers 34, 36, 38,and 40. As the sheet workpiece 22 enters the zero gap assembly 20, ittravels around the outer peripheral surface of the top or upper roller42 until it reaches the nip 46 defined between the top or upper roller42 and the bottom or lower roller 44. As previously noted, the fasterrotating bottom roller 44 pulls the sheet workpiece 22 through the nip46 while the slower rotating top or upper roller 42, as well as the millreduction process per se, effectively acts as a brake upon the lowersurface of the sheet workpiece 22, that is, the surface of the workpiece22 which is disposed in contact with the upper or top roller 42. Thus,the sheet 22 accelerates through the nip 46 defined between the rollers42 and 44, and is simultaneously milled and stretched to its finalizedpredetermined thickness dimension as the same passes through the nip 46.The exit bridle assembly 26 subsequently pulls the finished sheet 22 offfrom the bottom or lower roller 44 of the zero gap assembly 20, and itis therefore appreciated that in accordance with the method of thepresent invention, there is able to be achieved the production of athin, flat, oriented sheet 22 which is ready to be surface treatedand/or heat treated as desired, and sliced into thin straps as requiredor desired for use in strapping packages and the like in accordance withknown procedures.

It is again reiterated that the above-described apparatus and processproduce a significantly better quality strap than those able to beformerly produced in accordance with known or prior art apparatus andprocesses, as also exemplified and illustrated by means of the variousdata noted in the following table:

                  TABLE I                                                         ______________________________________                                                       SINGLE DRAW                                                                             ZERO GAP                                                            PROCESS   PROCESS                                              ______________________________________                                        Tensile Strength (KPSI)                                                                        45          64                                               Elongation (%)   25          13                                               Modulus (@ 2 & 5 KPSI)                                                                         400         963                                              Weld Strength (Lbs)                                                                            79          187                                              Weld Percent     55          89                                               Weld Equivalent (KPSI)                                                                         25          57                                               Split (In)       0.7         0.07                                             ______________________________________                                    

As can be appreciated from the table noted above, the zero gap methodproduces strapping which exhibits higher tensile strength, and astronger and higher percentage weld. Furthermore, splitting of thestrapping has been essentially eliminated while still achieving hightensile strength, whereas in accordance with known or prior artmanufacturing techniques and processes, as the tensile strength isincreased, increased splitting occurs and the percent of weld strengthdecreases. Still further, since the tensile strength of the strappingproduced in accordance with the zero gap process is approximately 1.47times the tensile strength of conventionally produced strapping, andsince the elongation of such strapping of the present invention isapproximately one-half that of the conventional strapping, better creepperformance is achieved. Such results present several market advantagesin connection with polypropylene material straps. More particularly, ifthe break strength of the strapping is a controlling factor inconnection with the particular application or use of the strapping, thenthe higher tensile strength of the material will permit the use orsubstitution of a strap which comprises only approximately seventypercent of currently used material or strapping. Similarly, if stiffnessis a controlling factor, characteristic, or attribute, the strappingproduced in accordance with the present invention is such as to bereadily capable of being reliably fed into a strapping machine by beingpushed around a guide chute of the strapping machine. Still yet further,if weld strength of the strapping is a controlling factor orcharacteristic, then strapping comprising less than one-half of thecurrently or conventionally used raw material will produce theequivalent joint strength.

The various properties resulting from the process of the presentinvention give significant flexibility of design to the strapping whichmay then be used in connection with a variety of applications. It isbelieved, for example, that the strapping produced by means of theprocess of the present invention exhibits a stronger bond across thegrain, while still being relatively easy to tear the strap across thegrain. Furthermore, the strap produced by means of the zero gap processof the present invention does not exhibit structual delamination as isoften characteristic of most prior art strapping. Since the strap of thepresent invention does not exhibit structural delamination, higher weldstrength is also obtained. As has also been noted hereinbefore, if apre-stretch step is performed between the entry bridle assembly and thezero gap assembly, or if a post-stretch step is performed between thezero gap assembly and the exit bridle assembly, the same overallcharacteristics are achieved as are achieved in accordance with theembodiment of the invention described in connection with the system andprocess of FIGS. 1-4. It is noted, however, that if a pre-stretchprocess step is performed upon the sheet workpiece 22, a higher tensilemodulus can be achieved, whereas if a post-stretch process step isperformed upon the finished sheeting 22, the material comprising thesheeting has a somewhat greater tendency toward fibrillation.

With reference again being made to the apparatus or system of FIG. 1, ithas been discovered that when the sheet workpiece 22 is conductedthrough the entry bridle assembly 32, and particularly as the workpiece22 approaches the upper milling and stretching roller 42 of the zero gapassembly 20, the upper or outer surface of the sheet workpiece 22, thatis, the surface thereof which is not disposed in contact with theperipheral surface of the upper or top roller 42 of the zero gapassembly 20, will tend to cool at a faster rate than the lower or innersurface of the sheet workpiece 22 which is disposed in contact with theperipheral surface of the upper or top roller 42 of the zero gapassembly 20 in view of the fact that the upper or outer surface of thesheet workpiece 22 is exposed directly to the ambient air. As a resultof such uneven or unequal cooling of the surfaces, or as a result of thedevelopment of such a temperature differential between the notedsurfaces of the sheet workpiece 22, the sheet workpiece 22 exhibitsdensity differentials throughout the thickness thereof whereby suchdensity differentials adversely affect some of the properties orcharacteristics of the sheet workpiece. Consequently, in order torectify, resolve, or eliminate such problem, an improved system orapparatus, generally indicated by the reference character 100, has beendeveloped and is shown in FIG. 5. The apparatus 100 is seen to comprisean entry bridle assembly 132 similar to the entry bridle assembly 32 ofthe embodiment of FIG. 1, and similarly, there is disclosed a zero gapassembly 120 which is similar to the zero gap assembly 20 of theembodiment of FIG. 1 and is seen to comprise upper and lower milling andstretching rollers 142 and 144, respectively. The entry bridle assembly132 is seen to comprise rollers 134, 136, 138, 140, and 141, all ofwhich are heated to an operative temperature of approximately 265° F.The upper or top roller 142 of the zero gap assembly 120 is heated to atemperature of approximately 290° F., and the lower or bottom roller 144of the zero gap assembly 120 is heated to a temperature of approximately250° F. In accordance with this embodiment of the present invention, inorder to maintain the outer surface of the sheet workpiece 122, that is,the surface of the sheet workpiece 122 which is not disposed in contactwith the peripheral or circumferential surface of the upper or toproller 142 of the zero gap assembly 120, at a temperature level which issubstantially equal to the temperature level of the inner surface of thesheet workpiece 122, that is, the surface of the sheet workpiece 122which is disposed in contact with the peripheral or circumferentialsurface of the upper or top roller 142 of the zero gap assembly 120, anadditional heated roller 143 is interposed between the last roller 141of the entry bridle assembly 132 and the upper or top roller 142 of thezero gap assembly 120. Roller 143 is heated to an operative temperatureof approximately 280° F. and accordingly it is noted that while thesurface temperature of the sheet workpiece 122 at a location just priorto entry into the entry bridle assembly 132, that is, at a position justimmediately upstream of the first entry bridle assembly roller 134, isapproximately 65° F., the surface temperature of the sheet workpiece 122at a location immediately downstream of the entry bridle assembly 132or, in other words, at the location interposed between the last entrybridle assembly roller 141 and the additional heated roller 143, isapproximately 219° F., the sheet workpiece 122 obviously having beensubstantially heated by means of the heated entry bridle assemblyrollers 134, 136, 138, 140, and 141. As a result of the further routingof the sheet workpiece 122 about the outer peripheral or circumferentialsurface of the additional heated roller 143, the outer surface of thesheet workpiece 122, that is, the surface of the sheet workpiece 122which will not be disposed in contact with the upper or top roller 142of the zero gap assembly 120, is heated still further such that at alocation interposed between the additional heated roller 143 and theupper or top roller 142 of the zero gap assembly 120, the temperature ofthe outer surface of the sheet workpiece 122 is approximately 242° F.This is somewhat higher than the temperature of the opposite or innersurface of the sheet workpiece 122 which was not directly heated bymeans of the additional heated roller 143, however, when the innersurface of the sheet workpiece 122 comes into contact with the upper ortop roller 142 of the zero gap assembly 120, such inner surface will beheated further. In addition, the outer surface of the sheet workpiece122 tends to cool due to the fact that such surface is not disposed incontact with the heated, upper or top roller 142 of the zero gapassembly 120 and that such surface is exposed to the ambient air.Consequently, at a location which is immediately upstream of the nip 146defined between the upper and lower rollers 142 and 144, respectively,of the zero gap assembly 120, both surfaces of the sheet workpiece 122will have approximately the same temperature which is approximately 236°F. In this manner, the density of the material comprising the sheetworkpiece 122 is substantially uniform across the thickness dimension ofthe sheet workpiece 122 whereby optimum properties can be achieved inthe finished sheet 122 which has by such time been simultanesouly milledand stretched as a result of being passed through the nip 146 definedbetween the zero gap assembly rollers 142 and 144. It is to be furthernoted that the outer surface of the finished sheet 122 which has justemerged from the nip 146 defined between the rollers 142 and 144, thatis, the surface of the finished sheet 122 which is not disposed incontact with the lower or bottom roller 144 has a temperature ofapproximately 290° F. the sheet 122 having experienced such a dramaticincrease in its temperature due to the working of the same within thezero gap assembly 120 and having undergone simultaneous milling andstretching within the zero gap assembly 120. After being route aroundthe outer peripheral or circumferential surface of the lower or bottomroller 144 of the zero gap assembly 120, the finished sheet 122 isconducted downstream for further processing thereof into elongatedpackaging straps, and at this location, the sheet 122 may exhibit asurface temperature of approximately 277° F.

It is to be emphasized that the importance of the provision of theadditional heated roller 143 is to, in effect, "overheat" the outersurface of the sheet workpiece 122 with respect to the inner surface ofthe sheet workpiece 122 to such a degree that such "overheating" willcompensate for the fact that such outer surface of the sheet workpiece122 will not be otherwise heated as will the inner surface of the sheetworkpiece 122 when the same is disposed in contact with the heated upperroller 142 of the zero gap, assembly 120, and that such outer surface ofthe sheet workpiece 122 will experience cooling thereof, which the innersurface of the sheet workpiece 122 will not, due to the fact that theouter surface of the sheet workpiece 122 is exposed to the ambient airas the inner surface of the sheet workpiece 122 is not exposed to theambient air due to its surface contact with the outer peripheral surfaceof the upper roller 142 of the zero gap assembly 120. In view of thesefactors, both surfaces will therefore exhibit substantially the samesurface temperatures at a point immediately upstream of the nip 146defined between the zero gap assembly rollers 142 and 144 whereby theimproved density profile across or throughout the thickness of the sheetworkpiece 122, and the resultant properties derived from or dependentupon such uniform density profile, are able to be achieved.

Continuing further, it is known that during processing of the sheetworkpiece 122, that is, by means of the, aforenoted milling andstretching thereof, the side edge portions of the sheet workpiece 122will tend to be thicker than the more centrally located portions of thesheet workpiece 122. The reason for this is that as the sheet workpiece122 is elongated in its longitudinal direction, the width and thicknessdimensions thereof are accordingly reduced with respect to the width andthickness dimensions of the original, non-worked, cast sheet workpiece122. In addition the different regions of the sheet workpiece 122 willact differently with respect to each other thereby leading to thedifferences in the thickness dimensions of the edge regions or portionsof the sheet workpiece 122 and of the central portions or regions of thesheet workpiece 122. For example, if the sheet workpiece 122 was dividedacross its width dimension into equal small regions, it would beappreciated that the central regions would be laterally confined orrestrained by neighboring regions upon opposite sides thereof, whereaswithin the edge regions of the sheet workpiece, such regions are onlyconfined or restrained in one lateral direction or upon one side thereofbecause the opposite side comprises a free edge or side. In a similarmanner, the upper and lower surfaces of the sheet workpiece are notconfined or restrained, and this is true for both the central and edgeportions or regions of the sheet workpiece. Consequently, when the sheetworkpiece undergoes goes elongation, the central regions of the sheetworkpiece cannot be reduced in width as much as the edge regions orportions because of the additional lateral constraints or confinementsimposed upon the central regions or portions, however, in order thateach region or portion retain its original volumetric value, even thoughthe volumetric configuration of a particular region or portion may bereorganized or rearranged, the thickness dimension of the centralregions or portions is reduced to a greater extent than the thicknessdimension of the edge portions or regions, or alternatively, thethickness dimension of the edge regions or portions is greater than thethickness dimension of the central regions of the sheet workpiece. Whenthe sheet workpieces used in connection with the present inventionusually have a width dimension of approximately twenty-four inches (24")after being simultaneously milled and stretched within the zero gapassembly 120, it has been found that the thicker edge portions occurupon the opposite sides of the oriented or finished sheet 122 for alateral or widthwise extent of approximately one and one-half inches(11/2").

In accordance with one of the objectives of the present invention, it isdesired to effectively reduce the lateral or widthwise extent of thethickened side edge portions or regions of the finished oriented sheet122, and it has been found that if the side, edge portions of the sheetworkpiece 122 were heated prior to the entry of the sheet workpiece 122into the nip 146 defined between the zero gap assembly rollers 142 and144, the lateral or widthwise extent of the thickened side edge portionsor regions of the finished oriented sheet 122 could be dramaticallyreduced whereby a substantially increased volume of the finishedoriented sheet 122, having a desired, uniform thickness or degree offlatness, could be achieved. In particular, it has been found that thelateral or widthwise extent of each thickened side edge portion or,region of the finished oriented sheet 122 can be significantly reducedfrom the aforenoted one and one-half inches (11/2") to approximately onehalf inch (1/2"). This reduction in the lateral or widthwise extent ofthe thickened side edge regions or portions of the finished orientedsheet 122 thus results in less waste or trim to be removed from thefinished oriented sheet 122 and concomitantly results in a larger amountor volume of finished oriented sheet 122 which may then be processedinto strapping material.

Briefly, as best understood, one reason that the aforenoted heating ofthe side edge portions of the sheet workpiece prior to entry into thenip region 146 defined between the zero gap assembly rollers 142 and 144would result in the reduction of the widthwise or lateral extent of thethickened side edge portions of the finished oriented sheet 122 is thatwithin the sheet workpiece, tensile forces or loads act inwardly towardthe central region of the sheet workpiece, and the ability to resisttensile loads is a function of temperature. Consequently, when you heatthe side edge portions of the sheet workpiece 122, the resistance to thetensile loads is decreased within those regions or portions whereby suchouter or side edge portions or regions tend to expand in the lateral orwidthwise direction such that the side edge portions or regions of thefinished oriented sheet 122 tend to become thinner with a somewhatcorresponding increase in the overall width of the finished orientedsheet 122 as compared to a similarly finished, oriented sheet 122 whichwas not pre-heated within the side edge portions or regions thereof.With reference therefore again being made to FIG. 5, it is seen that inaccordance with the principles of the present invention, andfurthermore, in accordance with the foregoing discussion, the apparatusor system 100 has incorporated therein radiant, infrared heaters whichare adapted to pre-heat the edge portions of the sheet workpiece 122prior to its entry into the nip 146 defined between the upper and lowerzero gap assembly rollers 142 and 144. In particular, a pair of heaters188 are effectively disposed with respect to the upper zero gap assemblyroller 142 so as to envelope the same for an arcuate or circumferentialextent of 180° and the heaters 188 are also axially disposed along theaxis of the roller 142 so as to be optimally positioned with respect tothe oppositely disposed edge portions of the sheet workpiece 122. Eachof the heaters 188 is approximately two and one-half inches (21/2") wideand is operated at 240 VAC with a generated power of 2500 Watts. Inaddition, a second pair of heaters 190, which are linear inconfiguration, are operatively connected to the upstream end of theheaters 188, respectively, as viewed in the direction of travel of thesheet workpiece 122 toward the zero gap assembly 120. These heaters 190are similarly two and one-half inches (21/2") wide, however, they mayonly need to generate a power of 1900 Watts at 240 VAC. Still further,as yet an additional option or alternative, the sheet workpiece 122 maybe initially preheated within the entry bridle assembly 132 by means ofthe disposition of additional heaters 192 which are similar to theheaters 188 in that they have the arcuate configuration enveloping, forexample, the upper entry bridle assembly roller 138 over acircumferential extent of 180° however such heaters 192 need onlygenerate 1500 Watts power at 240 VAC, and are of the same width.

With reference now being made to FIG. 6, once the sheet workpiece 122has been milled and stretched within the zero gap assembly 120, and whenthe side edge portions thereof have been preheated in accordance withthe foregoing discussion and disclosures of the heaters 188, 190, and192, the finished sheet 122 is conducted through an exit bridle assemblygenerally indicated by the reference character 194 and a flame treatbridle assembly 196 which is conventional. As has been notedhereinbefore in connection with the system of FIG. 1, the lineal surfacevelocity of each entry bridle assembly roller 34, 36, 38, and 40 isessentially the same as that of the upper roller 42 of the zero gapassembly, and the lineal surface velocity of each exit bridle assemblyroller 76, 78, 80. 82, 84, and 86 is essentially the same as that of thelower roller 44 of the zero gap assembly 20, and the lineal surfacevelocity of the lower roller 44 of the zero gap assembly 20 is withinthe range of 7-12 times as fast as the lineal surface velocity of theupper roller 42 of the zero gap assembly 20, with a lineal surface ratioof approximately 9.5 being preferred. As an alternative to the foregoinghowever, the exit bridle assembly 194 rollers may be operated at alineal surface velocity which is greater than the lineal surfacevelocity of the lower zero gap assembly roller 144 by an amount which iswithin the range of two percent (2%) to twenty percent (20%). It is tobe noted that tensile strength increases with a corresponding increasein exit draw, the width of the finished sheet correspondingly decreases,however, the finished sheet becomes more split sensitive. Consequently,the exit draw values must be chosen or selected from an optimum balanceof properties point of view, and accordingly, it has been found that anexit draw value of two percent (2%) to ten per cent (10%) is quitesatisfactory.

With reference still being made to FIG. 6, and in accordance withfurther processing of the finished sheet 122, edge trim knives 198 arelocated within the processing line at a position interposed between theexit bridle assembly 194 and the flame treat bridle assembly 196. Theedge trim knives 198 serve to trim the thickened edge portions of thefinished sheet 122 such that the trimmed edge portions of the finishedsheet 122 are then removed from the processing line as at 200 while thetrimmed finished sheet 122 is then conducted onto storage rolls 202 forsubsequent use in actually fabricating thermoplastic strapping.

In accordance with the foregoing, edge heating techniques have beendescribed in connection with the fabrication of finished sheets 122which exhibit increased degrees of flatness or an increase amount ofuniformly flat finished sheet across the width dimension whereby acorrespondingly increased amount of strapping can be produced from suchfinished sheet 122. Increased flatness or increased amounts of uniformlyflat finished sheets across the width dimensions thereof can also beachieved in accordance with other manufacturing techniques. For example,with reference being made to FIGS. 7A and 7B, there is shown, in effect,front view profiles of two differently contoured mill rollers which maybe incorporated within the zero gap assembly 20 of the system of FIG. 1or the zero gap assembly 120 of the system of FIG. 5. As can beappreciated from the contoured mill rollers 204 and 206 of FIGS. 7A and7B, respectively, the central portions of the rollers 204 and 206 areeffectively concave as shown at 208 and 209, respectively, while thelateral side edge portions 211 and 213 have straight or horizontalconfigurations or contours In this manner, when the sheet workpiece 22or 122 is milled and stretched within the nip 46 or 146, the edgeportions of the sheet workpiece 22 or 122 will be worked to a greaterdegree than the central portions of the sheet workpiece 22 or 122whereby the edge portions of the sheet workpiece 22 or 122 will berendered thinner. Upon being processed within the nip 46 or 146, theedge portions of the sheet workpiece will also undergo thickening tosome extent as described hereinbefoe, whereby the finalized edge regionsof the sheet workpiece or finished sheet 22 or 122 will have the desireddegree of thinness and uniformity of flatness as may be similarlyachieved by means of the aforenoted edge heating techniques. As can befurther appreciated from FIGS. 7A and 7B, the primary difference betweenthe contours or configurations of the mill rollers 204 and 206 residesin the fact that in the case of the roller 204 of FIG. 7A, the centralportion thereof has a cylindrical contour as at 215 while the adjacentaxial portions thereof have tapered or frustoconical configurations asat 217. Finally, the outermost end or edge portions are cylindrical asat 219, the diametrical extent of cylindrical portions 219 being,however, greater than the diametrical extent of central cylindricalportion 215. Similarly, in the case of the roller 206 of FIG. 7B, thecentral portion 209 has a concave arcuate configuration while theoutermost end portions 221 are cylindrical and have diametrical extentsgreater than the central portion 209.

With reference lastly being made to FIGS. 8A, 8B, and 9, anothermanufacturing technique which may be employed in order to achieveincreased flatness or increased amounts of uniformly flat finishedsheets across the width dimensions thereof involves the extrusion of thesheet workpieces 22 or 122 from particularly configured extrusion dies223 and 225 which are respectively shown in FIGS. 8A and 8B. As can beappreciated, each of the dies 223 and 225 has, in effect, a die mouth227 and 229 which has a trapezoidal configuration which thereforeproduces sheet workpieces 22 and 122 which likewise have trapezoidalcross-sectional configurations, or in other words, the edge portions 231and 233 will be substantially thinner than the central portions of thesheet workpieces 22 and 122. In this manner, when these trapezoidalconfigured sheet workpieces 22 and 122 are milled and stretched, theiredge portions will tend to thicken somewhat whereby the resulting edgeportions will have thickness dimensions substantially the same as thecentral portions of the sheet workpieces 22 and 122 such that asubstantially uniform thickness dimension is achieved acrosssubstantially the entire width of the sheet workpieces 22 or 122 exceptof course in the outermost edge regions as has been noted hereinbefore.It will be further appreciated that the only significant differencebetween the dies 223 and 225 is that die 223 produces a sheet workpiece22 or 122 which has a greater thickness dimension than a similarworkpiece 22 or 122 that may be produced by means of die 225. In FIG. 9,a set of casting rollers 235, 237, and 239 are disclosed as beingarranged in a vertical array. The upper and lower rollers 235 and 239each have configurations somewhat similar to those of mill rollers 204and 206, while the central casting roller 237 has a cylindricalconfiguration. Consequently, it can appreciated that the spaces 241 and243 defined respectively between the casting rollers 235 and 237, and237 and 239, have, in effect, trapezoidal configurations such that whenthe sheet workpieces 22 or 122 are extruded from the die mouths 227 or229, the workpiece 22 or 122 is initially routed about the castingrollers so as to be inserted into the space 241, wrapped about theroller 237, and withdrawn from space 243. Consequently, it is seen thatthe trapezoidal configuration of space 243 is, in effect, up-side-downwith respect to the trapezoidal configuration of space 241. The sheetworkpiece 22 or 122 may then be conveyed further toward the zero gapassembly 20 or 120 for processing in accordance with the techniques ofthe present invention.

Thus, it may be seen that in accordance with the foregoing andparticularly the manufacturing techniques and apparatus developed inaccordance with the principles of the present invention, the finishedmilled and stretched sheets exhibit improved density, and thereforeoperative or field use properties, and in addition, the sheets alsoexhibit improved degrees or uniformity of flatness whereby increasedvolumes of thermoplastic strapping may be produced or derived therefrom.

Obviously, many modifications and alterations may be derived ordeveloped in light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims, the presentinvention may be practiced otherwise than as specifically describedherein.

What is claimed as new and desired to be protected by means of LettersPatent of the UNITED STATES of AMERICA, is:
 1. Apparatus forsimultaneously milling and stretching a solid sheet workpiece whichpasses through said apparatus in a predetermined direction of travel,comprising in combination:a pair of opposed rollers defining a niptherebetween and through which said solid sheet workpiece is passed soas to be simultaneously milled and stretched as said solid sheetworkpiece passes through said nip in a single pass; means for drivingsaid opposed rollers in opposite directions and at sufficientlydifferent lineal surface velocities for simultaneously milling andstretching said solid sheet workpiece within said nip defined betweensaid pair of opposed rollers; and means, disposed adjacent to andupstream of a first upstream one of said pair of opposed rollers, asconsidered in said direction of travel, for heating an outer surfaceportion of said solid sheet workpiece, which is not disposed in contactwith said first upstream one of said pair of opposed rollers and whichis located at a location upstream of said nip defined between said pairof opposed rollers, to a temperature level which is greater than thetemperature level of an inner surface portion of said solid sheetworkpiece which is disposed in contact with said first upstream one ofsaid pair of opposed rollers such that when said outer and inner surfaceportions of said solid sheet workpiece reach said nip defined betweensaid pair of opposed rollers, said temperature levels of said inner andouter surface portions of said solid sheet workpiece will besubstantially the same whereby the density of said solid sheet workpieceacross the thickness dimension of said solid sheet workpiece will besubstantially uniform.
 2. Apparatus as set forth in claim 1,wherein:said heating means comprises a heated roller disposed up-streamof said pair of opposed rollers.
 3. Apparatus as set forth in claim 2,wherein:said apparatus further comprises an entry bridle assembly; andsaid heated roller is interposed between said pair of opposed rollersand said entry bridle assembly.
 4. Apparatus as set forth in claim 2,further comprising:means for heating said heated roller to a temperaturelevel of approximately 280° F. so as to, in turn, heat said outersurface of said sheet workpiece to a temperature level of approximately242° F.
 5. Apparatus as set forth in claim 4, further comprising:meansfor heating said first upstream one of said pair of opposed rollers to atemperature level of approximately 290° F. so as to, in turn, heat saidinner surface portion of said sheet. workpiece such that both said outerand inner surface portions of said sheet workpiece will havesubstantially the same temperature level of approximately 236° F. at alocation immediately upstream of said nip defined between said pair ofopposed rollers.
 6. Apparatus as set forth in claim 1, wherein:saidmeans for driving said pair of opposed rollers comprises means fordriving a second downstream one of said pair of opposed rollers at alineal surface velocity which is within the range of 7-12 times greaterthan the lineal surface velocity of said first, upstream one of saidpair of opposed rollers.
 7. Apparatus as set forth in claim 6,wherein:said means for driving said pair of opposed rollers comprisesmeans for driving said second downstream one of said pair of opposedrollers at a lineal surface velocity which is 9.5 times said linealsurface velocity of said first upstream one of said pair of opposedrollers.
 8. Apparatus for simultaneously milling and stretching a sheetworkpiece which passes through said apparatus in a predetermineddirection of travel, comprising:a pair of opposed rollers defining a niptherebetween and through which said sheet workpiece is passed so as tobe simultaneously milled and stretched as said sheet workpiece passesthrough said nip in a single pass; means for driving said opposedrollers in opposite directions and at sufficiently different linealsurface velocities for simultaneously milling and stretching said sheetworkpiece within said nip defined between said pair of opposed rollers;and heater means disposed about opposite edge portions of said sheetworkpiece for heating said opposite edge portions of said sheetworkpiece at a location upstream of said nip defined between said pairof opposed rollers so as to increase the degree of flatness of saidsimultaneously milled and stretched sheet workpiece across the width ofsaid simultaneously milled and stretched sheet workpiece as saidsimultaneously milled and stretched sheet workpiece passes through saidnip defined between said pair of opposed rollers.
 9. Apparatus as setforth in claim 8, wherein:said heating means comprises radiant, infraredheaters.
 10. Apparatus as set forth in claim 9, wherein:each one of saidradiant, infrared heaters has a width of approximately two and one-halfinches.
 11. Apparatus as set forth in claim 8, wherein:said heatingmeans partially envelop a first upstream one of said pair of opposedrollers as considered in said direction of travel of said sheetworkpiece through said apparatus.
 12. Apparatus as set forth in claim11, further comprising:an entry bridle assembly comprising entry bridlerollers; and said heating means comprises a linear portion for heatingside edge portions of said sheet workpiece along a linear extent whichextends between said entry bridle assembly and said upstream one of saidpair of opposed rollers, and an arcuate portion for heating saidopposite side edge portions of said sheet workpiece along an arcuateextent which is routed about said first upstream one of said pair ofopposed rollers.
 13. Apparatus as set forth in claim 12, wherein:saidarcuate portion of said heating means has a circumferential extent ofapproximately 180°.
 14. Apparatus as set forth in claim 8, furthercomprising:an entry bridle assembly comprising entry bridle rollers;said heating means partially envelops one of said entry bridle rollers.15. Apparatus as set forth in claim 14, wherein:said heating means hasan arcuate configuration which has a circumferential extent ofapproximately 180° so as to partially envelop said one of said entrybridle rollers.
 16. Apparatus as set forth in claim 8, wherein:saidmeans for driving said pair of opposed rollers comprises means fordriving a second downstream one of said pair of opposed rollers, asconsidered in said direction of travel, at a lineal surface velocitywhich is within the range of 7-12 times greater than the lineal surfacevelocity of a first upstream one of said pair of opposed rollers. 17.Apparatus as set forth in claim 16, wherein:said means for driving saidpair of opposed rollers comprises means for driving said seconddownstream one of said pair of opposed rollers at a lineal surfacevelocity which is 9.5 times said lineal surface velocity of said firstupstream one of said pair of opposed rollers.
 18. Apparatus forsimultaneously milling and stretching a sheet workpiece which passesthrough said apparatus in a predetermined direction of travel,comprising:a pair of opposed rollers defining a nip therebetween andthrough which said sheet workpiece is passed so as to be simultaneouslymilled and stretched as said sheet workpiece passes through said nip ina single pass; and means for driving said opposed rollers in oppositedirections and at sufficiently different lineal surface velocities forsimultaneously milling and stretching said sheet workpiece within saidnip defined between said pair of opposed rollers; said opposed rollerseach having a configuration comprising concave central portions andcylindrical edge portions so as to provide said sheet workpiece withthinner edge portions than central portions when said sheet workpiece isconducted through said nip defined between said pair of opposed rollerssuch that an increased degree of flatness of said simultaneously milledand stretched sheet workpiece across the width of said simultaneouslymilled and stretched sheet workpiece, as said simultaneously milled andstretched sheet workpiece passes through said nip defined between saidpair of opposed rollers, is achieved.
 19. Apparatus as set forth inclaim 18, wherein:said central portions of said opposed rollers arearcuate in configuration.
 20. Apparatus as set forth in claim 18wherein:said central portions of said opposed rollers have a trapezoidalconfiguration.
 21. Apparatus as set forth in claim 18, wherein:saidmeans for driving said pair of opposed rollers comprises means fordriving a second downstream one of said pair of opposed rollers, asconsidered in said direction of travel, at a lineal surface velocitywhich is within the range of 7-12 times greater than the lineal surfacevelocity of a first upstream one of said pair of opposed rollers. 22.Apparatus as set forth in claim 21, wherein:said means for driving saidpair of opposed rollers comprises means for driving said seconddownstream one of said pair of opposed rollers at a lineal surfacevelocity which is 9.5 times said lineal surface velocity of said firstupstream one of said pair of opposed rollers.
 23. Apparatus forsimultaneously milling and stretching a sheet workpiece which passesthrough said apparatus in a predetermined direction of travel,comprising:die means for extruding a sheet workpiece; a pair of opposedrollers defining a nip therebetween and through which said sheetworkpiece is passed so as to be simultaneously milled and stretched assaid sheet workpiece passes through said nip in a single pass; and meansfor driving said opposed rollers in opposite directions and atsufficiently different lineal surface velocities for simultaneouslymilling and stretching said sheet workpiece within said nip definedbetween said pair of opposed rollers; said die means having apredetermined cross-sectional configuration as defined across thethickness dimension of said sheet workpiece such that said extrudedsheet workpiece has thinner edge portions than central portions and suchthat an increased degree of flatness of said simultaneously milled andstretched sheet workpiece across the width of said simultaneously milledand stretched sheet workpiece, and as considered with respect to thethickness dimension of said sheet workpiece, after said simultaneouslymilled and stretched sheet workpiece has passed through said nip definedbetween said pair of opposed rollers is achieved.
 24. Apparatus as setforth in claim 3, wherein:said predetermined cross-sectionalconfiguration of said die means comprises a trapezoid.
 25. Apparatus setforth in claim 24, further comprising:a plurality of casting rollersdisposed within a vertical array and defining spaces therebetween havingtrapezoidal configurations for accommodating trapezoidal-configuredsheet workpieces produced by said die means.
 26. Apparatus as set forthin claim 23, wherein:said means for driving said pair of opposed rollerscomprises means for driving a second downstream one of said pair ofopposed rollers, as considered in said direction of travel, at a linealsurface velocity which is within the range of 7-12 times greater thanthe lineal surface velocity of a first, upstream one of said pair ofopposed rollers.
 27. Apparatus as set forth in claim 26, wherein:saidmeans for driving said pair of opposed rollers comprises means fordriving said second, downstream one of said pair of opposed rollers at alineal surface velocity which is 9.5 times said lineal surface velocityof said first upstream one of said pair of opposed rollers.